Learning how to diagnose bad flying weather

Learning how to diagnose bad flying weather

Lightning poses hazard to rockets,
aircraft

One of a series of stories covering the
quadrennial International Conference on Atmospheric Electricity,
June 7-11, 1999, in Guntersville, Ala.

June 8, 1999: In the 1950s, a research scientist
discovered that small rockets he launched to study lightning
would sometimes trigger lightning. That heralded a new hazard
for the space age, the problem of space launchers falling prey
to lightning before they even reached space.

But no one worried much about the problem until Nov. 19, 1969,
when Apollo 12 was struck twice by lightning in the 36.5 seconds
after liftoff, when it was just 1.8 km (6,000 ft) high.

"That opened our eyes to the hazards that lightning could
pose to a spacecraft traveling through an electrified cloud,"
said John Willett, a retired research scientist with the U.S.
Air Force Research Laboratory outside Boston. The subject was
reemphasized in 1987 when an Atlas Centaur was struck by lightning
that damaged its electronics and caused an engine to steer the
rocket off course.

Willett was an invited speaker Monday afternoon at the International
Conference on Atmospheric Electricity being held this week in
Guntersville.

"What are the conditions in the atmosphere
that are conducive to triggering lightning?" Willett asked.
"If we know what they are, we have a better chance of avoiding
them."

For now, the conditions are not fully known, so Willett and
scientists at other institutions have developed a rocket program
to find out. They use a small sounding rocket that uses surplus
2.75-inch-diameter Mighty Mouse motors designed for launch from
attack aircraft. With a diagnostic payload designed for the rocket,
the complete package is 6 feet tall.

Willett said the rocket is launched into electrified clouds
to measure conditions, principally how the electric potential
or voltage changes with altitude. Then a small rocket, trailing
a copper wire, is launched to trigger a lightning flash.

"When we do it we take great care to protect ourselves,"
Willett said. The launch team and their equipment are inside
a trailer that is modified to serve as a Faraday cage, a sort
of electrical isolation chamber that conducts any stray lightning
around them.

"I don't anticipate any way of preventing the lightning
from striking the spacecraft," Willett said. "We'll
have to take measurements in advance and say it is safe to launch,
or it is not safe."

For the time being, launch sites like Cape Canaveral and Kennedy
Space Center operate with lightning safety constraints that cost
them a few launch opportunities, but minimize the chance of getting
struck.

"There's a very definite set of launch commit criteria
imposed on national launch ranges," Willett said. "We
are quite sure that it's safe before a rocket launches. But it
leads to a reduction in launch opportunities."

Aircraft pose a different challenge. Most aircraft are well
shielded against lightning because of the design of their metal
airframes. Lightning is conducted around the skin and continues
on its way in most cases. But as designers move to lightweight
composites that don't conduct electricity, they face an increased
risk of damage to the aircraft.

Willett noted that some strikes have splintered the plastic
or rubberized radar domes on the nose of some aircraft. "You
don't want that happening to wings or other major structures,"
he said. Further, the electronics that really control the aircraft
will have to be shielded so they don't blank out for several
seconds, as happened when Apollo 12 was struck.